AN3168

Examples of negative power supplies

Doc ID 17193 Rev 1

13/20

3.2

Capacitive power supply

Figure 15 shows the schematic diagram of a capacitive negative power supply.

Figure 15.

Capacitive power supply

impedances and diode D2 voltage drop are neglected):

Equation 4

stress at power supply turn on and to avoid the overrating of the current protection of the

circuit. The standby power consumption is:

Equation 5

For a 230 V / 50 Hz mains voltage, a 60

Ω resistor R1 (typical value) and a 15 V zener diode,

the calculated average output current and standby power consumption versus different AC

capacitors given in Table 3.

The benefits of this solution are:

●

Low cost (for output current lower than 20 mA)

●

Small size

●

Easy to implement

Table 3.

Calculated output DC current and measured typical standby power

consumption

AC capacitor, C2

220 nF

470 nF

680 nF

1 µF

7.1 mA

15.3 mA

22.1 mA

32.5 mA

0.12 W

0.3 W

0.48 W

0.8 W

V

SS

C1

AC mains

V

DD

D1

D2

C2

R1

)

av

(

OUT

)

peak

(

mains

)

av

(

IN

I

V

2

C

f

2

I

=

×

×

×

=

)

av

(

IN

Z

2

)

av

(

IN

OFF

I

V

)

2

I

(

1

R

P

×

+

π

×

×

=